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Josephson Effects
Published in David A. Cardwell, David C. Larbalestier, I. Braginski Aleksander, Handbook of Superconductivity, 2023
Tunnel junctions usually consist of electrode layers of superconductor and/or normal metal separated by an insulating barrier, although it is important to note that other structures such as grain boundaries may exhibit tunnelling phenomenon [8]. The insulator may be another material sandwiched between the electrodes or may be a vacuum. The transmission of electrons through the barrier is determined by a transmission coefficient D, which according to elementary quantum mechanics should be of the form exp[-l√(2mΓ/ħ2)], where Γ is the barrier height, l the barrier width, m the electron mass and ħ Planck's constant. If the insulator is simply a vacuum layer, the barrier height Γ is simply the work function of the metal, whereas, if it consists of a dielectric layer, Γ is fixed by the bandgap in the barrier's electronic structure. A number of factors may complicate this simple picture, but the overall features are generally preserved and as a rule of thumb we can take a transmission probability D2 to be approximately given by exp(–l) if l is measured in Å [9].
Transfer of Single Electrons and Single Cooper Pairs in Nanojunction Circuits
Published in Günter Mahler, Volkhard May, Michael Schreiber, Molecular Electronics, 2020
Michel H. Devoret, Daniel Esteve, Cristian Urbina
There exists, however, a solid-state device in which electric charge flows in a discrete manner. It consists of two metallic electrodes separated by an insulating layer so thin that electrons can traverse it by the tunnel effect (5) (Fig. 1). Tunnelling can be considered as an all-or-nothing process because electrons spend a negligible amount of time under the potential barrier corresponding to the insulating layer (6,7). If one applies a voltage V to such a tunnel junction, electrons will randomly tunnel across the insulator at a rate given by V/eRt, where the tunnel resistance Rt is a macroscopic parameter of the junction that depends on the area and thickness of the insulating barrier. Apart from allowing the tunnel effect, the two facing electrodes behave as a capacitor whose capacitance C is the other macroscopic parameter of the junction. It is important to stress that the transport of electrons in a tunnel junction and in a metallic resistor are fundamentally different, even though the current–voltage characteristic is linear in both cases. Charge flows continuously along the resistor, whereas it flows across the junction in packets of e. Obviously, a tunnel junction provides the means to extract electrons one at a time from an electrode. With a single voltage-biased tunnel junction, however, it is not possible to control the instants at which electrons pass from the upstream electrode to the downstream electrode, because of the stochastic nature of tunnelling. A further ingredient is needed.
Graphene-Based Single-Electron Transistors
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2020
SETs, take advantage of an inherently quantum mechanical effect, that is tunneling, where a particle will tunnel through an energy barrier that it cannot overcome classically. Different ways to achieve a tunneling junction exist, ranging from narrow channels that only permit tunneling currents through gateable junctions to tunnel junctions created through thin insulator layers [21–23]. Figure 11.4a,b show the scanning electron microscope (SEM) image and the circuit scheme of a graphene SET. The SET consists of an island or “dot” separated from the source and drain by two tunnel junctions. By changing the voltage on the plunger gate it is possible to control the electrochemical potential of the dot and the number of electrons on it. The barrier gates can be used to change the tunnel couplings Γs and Γd to the source and drain, respectively.
Performance investigation of nanoscale reversible logic gates designed with SE-TLG Approach
Published in International Journal of Electronics, 2020
The Single Electronic Devices (SEDs) (Averin & Likharev, 1991; Likharev, 1999; Schupp, 2017; Singh et al., 2007) are one of the emerging nanodevices which enable more compact, low power-consuming solution for the logical circuit designs. They work in the principle of tunnelling of electrons through the tunnel junction. The modelling (Abu El-Seoud et al., 2007; Ghosh et al., 2017; Ghosh & Sarkar, 2018a; Hasaneen et al., 2011; Radwan et al., 2015; Wu & Lin, 2003) of Single Electron Transistor (SET) (Kastner, 2000; Matsumoto, 1999; Matsumoto et al., 1996; Sui et al., 2010) and logic circuit simulation (Korotkov, 1999; Sharifi & Ahmadian, 2018) using them has already gained much popularity for its lucrative features. Previously a Single-electron transistor (SET)-based implementation of the different reversible logic gates such as NOT, Feynman and Toffoli has been proposed (Khan, 2015), but in that work the SET was considered as the pull-down network only and for the pull-up network part high-valued resistors were used. Thus, the power consumption of the circuit was in the nanowatt range. The threshold logic-based approach altogether with the basic SED principle makes it possible to reduce the power consumption further in the range of picowatts and thus enhances the performance of designed gates. Here the Single-electron linear threshold logic gate (SE-TLG) (Ghosh et al., 2019; Ghosh & Sarkar, 2018b; Lageweg et al., 2001, 2003) based implementation of the different reversible gates and their performance analysis is elaborated. A new reversible gate PE1, which is suitable for three-bit even parity generating circuits, is also proposed in this.